In recent years the escalating cost of fuel, particularly natural gas, has caused a significant change in the manner in which gas fuel burners have been operated. When the cost of natural gas was relatively low, many gas operated appliances used for space heating were operated with pilot lights that were continuously burning and which were monitored by a thermocouple or other simple heat responsive safety device. These types of systems were generally referred to as standing pilot burner systems.
The standing pilot burner system uses a small amount of fuel continuously, but was a very inexpensive type of ignition system that proved to be very reliable. With the advent of the rapidly rising cost of natural gas, the use of standing pilots has become of questionable economic value. In some places the use of standing pilot configurations in new installations has been legislated out of existence. To replace the standing pilot systems, new styles of electronically controlled and spark ignited pilot systems have become common. These new types of systems normally use a spark generator to ignite natural gas flowing from a pilot burner. Once the natural gas is ignited, the pilot is then in turn used to ignite a main burner. The monitoring of the pilot flame is typically accomplished in these systems by the well known technique of flame rectification sensing. In flame rectification sensing, a voltage is applied between a flame rod and the pilot burner and is capable of sensing the presence or absence of flame by a change in conduction of current through a circuit that includes the flame. The spark ignited type of pilot system typically utilizes a relaxation type of oscillator to generate the spark and then uses relays that are controlled by soild state gated switches, such as, silicon controlled rectifiers for control of fuel to the pilot burner and to the main burner. These systems are susceptible of false operation by the generation of electrical noise or interference that improperly gates the solid state switches. The spark generator is a primary source of noise and can falsely gate or trigger the soild state switches thereby creating a system operation that is undesirable, and even possibly unsafe. The filtering of this type of electrical noise, and the safe operation of the solid states switches has become a significant problem.